My De Rosa Merak has a carbon fiber seat post secured by a wedge type clamp. When I first started riding it (I bought the bike used), the seatpost kept slipping. I completely cleaned the seat post and the seat tube and used Finish Line Fiber Grip Carbon Fiber Bicycle Assembly Gel (affiliate link). The seat post still slipped when I torqued the bolt to the max 6 N⋅m.
I don’t know if I imagined seeing it done before but I thought that a piece of sandpaper glued to the back of the seatpost might help.
I completely cleaned the back of the seat post and using wood glue, I glued a piece of 120 grit sandpaper about 2.5″ long and 3/4″ wide to the back of the seat post. I aligned the top edge of the sandpaper just below the top tube, where the clamp applies pressure. The seat post hasn’t slipped again; maybe it works as a shim and it definitely adds more friction.